A method for the calculation of improved band gaps in the crystal orbital formalism

A computational method for the calculation of improved band gaps in the crystal orbital (CO) formalism based on the Hartee-Fock (HF) procedure is developed. The approach makes use of fractional k-averaged band occupational numbers in the band structure technique. An electron in the recalculated conduction band(s) feel the averageed potential of (N — 1) electrons. The method is applied to the LiH polymer. Semiempirical INDO CO calculations lead to an ?(k) lowering of nearly 3 eV for the improved conduction band in comparison to the dispersion curve of the standard variational approach. The two ?(k) curves are characterized due to slightly different dispersions.     

蛋白质晶体结构刚体优化的新方法

将多元函数的变尺度法与两电子密度图的相关系数相结合,得到一种能够对蛋白质晶体结构进行刚体优化的新方法.初步测试结果表明,该方法能够明显地改善待测晶体的初始模型在晶胞中的取向和位置.而与最大似然方法相比,该方法可能更适用于搜索空间中存在大量局域极值点的情况. 关键词： 蛋白质晶体结构 刚体优化 多元函数的变尺度方法 电子密度图的相关系数     

A diagrammatic method for the calculation of moments of operators and its application to nuclear spectroscopy

The physical content of the well-known Lanczos method of tridiagonalisation is analysed in the framework of nuclear spectroscopy of low-lying states. The formal calculations are based on diagrammatic methods. Their link with other methods and their possible use in other fields of spectroscopy are briefly considered.     

A general substructure synthesis method for the dynamic simulation of complex structures

In this paper a general substructure synthesis method is developed for the dynamic analysis of complex flexible structures. The motion of each substructure is represented by a given number of substructure admissible functions. Substructure admissible functions are often low-order polynomials, and hence computationally easy to work with. The otherwise disjoint substructures are connected together to form a whole structure by imposing approximate geometric compatibility conditions by means of the method of weighted residuals. The behavior of the estimated eigenvalues obtained by the substructures synthesis method can be ascertained by means of a bracketing theorem. The estimated eigenvalues do converge to the actual eigenvalues of the original structure, although it is necessary to consider two limiting processes, one in which the number of substructure admissible functions is increased and the other in which the number of internal boundary weighting functions is increased.     

A method of moments for the optical Jahn-Teller problem

A Method of moments is presented for optical transitions to Jahn-Teller states. The Condon and a modified Born-Oppenheimer approximation have been adopted. On the basis of these 2 principles, the method is exact. Specific orthonormality and closure properties are derived for Jahn-Teller states, which are essential for performing the calculation. As illustrative examples the pure Jahn-Teller casesE-e andT-t have been calculated in the strong coupling limit. Each of these cases leads to specific combinatorial problems and to an application of Wick's theorem. The latter is common to both cases. Comparison with the semi-classical calculation reveals the identity of the results for theE- e system.     

A novel method for measurement of concentration using two dimensional photonic crystal structures

A novel method to measure the concentration of sugar, sodium chloride and alcohol, in their aqueous solution by using two dimensional photonic crystal structures is presented. This measurement is very accurate as the principle is based on variation of photonic band gap with respect to concentration. Photonic band gap here is computed using plane wave expansion method.     

Predicting crystal structures: the Parrinello-Rahman method revisited

By suitably adapting a recent approach [A. Laio and M. Parrinello, Proc. Natl. Acad. Sci. U.S.A. 99, 12 562 (2002)]] we develop a powerful molecular dynamics method for the study of pressure-induced structural transformations. We use the edges of the simulation cell as collective variables and define a metadynamics that drives the system away from the local minimum towards a new crystal structure. In contrast to the Parrinello-Rahman method, our approach shows no hysteresis, and crystal structure transformations can occur at the equilibrium pressure. We illustrate the power of the method by studying the pressure-induced diamond to simple hexagonal phase transition in a model of silicon.     

Molecular orbital calculation of some vibrational properties of the complex between HCN and HF

Ab initio LCAO-SCF calculations with an extended basis set are used to predict the geometry and various vibrational properties of the hydrogen bonded complex between HCN and HF. The complex is predicted to be completely linear, HF being the proton donor. The frequencies of the intermolecular vibrations are in good agreement with experiment. The intramolecular frequency shifts and absorption intensity changes are also determined. The large increase in the intensity of the HF stretching frequency in the complex is found to be due to both charge transfer and polarization forces.     

A quark shell model calculation of nuclear magnetic moments

The “Schmidt lines” for nuclear magnetic moments are calculated in a quark shell model. In the pairing scheme proposed by Petry et al., the single-nucleon magnetic moment is simply related to the single quark value. The isoscalar part is essentially the same as that of the conventional Schmidt moment while the isovector part increases too rapidly as the angular momentum increases. Possibilities of improving the model are briefly discussed.     

Hybrid holographic-numerical method for modal analysis of complex structures

This paper presents a hybrid holographic-numerical method for modal analysis of complex structures. A continuous structure is first lumped into a number of discrete elements to form an elastically connected lumped linear system. The matrix of influence coefficients of the lumped linear system are then determined by exerting a static load to the element centers and measuring the corresponding whole-field displacement using digital holographic interferometry. The eigenvalues and eigenvectors of the influence coefficients, which in a physical sense represent the natural frequencies and mode shapes of the structure, are then calculated using the numerical method. A major advantage of the proposed hybrid method is that it is not necessary to know the Young's modulus, the Poisson's ratio of the material and the boundary conditions, as the displacement field measured by the optical method has automatically reflected the real boundary conditions and the material properties, which makes this method particularly useful for studying objects made from anisotropic materials such as composites. Another advantage of the proposed method is that structures of any complex and irregular shape will not increase the complexity of the characterization process. The proposed is also suitable for experimentally validating the modal analysis results from finite element method models.     

Comments on the method of moments applied to the calculation of optical absorption line shapes

In order to check reliability of the method of moments proposed by Sati et al., the line shape for an A_1g → T_1μ transition (plus T_2g-mode) is investigated in the high temperature limit. This method is applicable even when the original function has a singular point. It is essential to adopt an appropriate generating function which has the same asymptotic behavior as the original function.     

Clusters and bulk beryllium: A molecular orbital versus crystal orbital study

The quasirelativistic CNDO/1 and EHT methods have been applied to investigate the electronic structure of {Be}₁₂₅, {Be}₂₁₆ and {Be}₃₄₃ clusters generated by a multiplication of the unit cell. The DOS profiles and their projections were calculated. These data were correlated with the periodic crystal orbitals of the EHT and ab-initio quality, respectively, as well as with the ASW method.We acknowledge C. Roetti for making the New version of CRYSTAL 92 available and thank P. C. Schmidt for ASW computer code. This work is in part supported by the Slovak Grant Agency for Science (research grant No. 4340).     

A molecular orbital calculation of the charge distribution of CS2

Equations are derived for the singlet phase distribution for a dense classical fluid using the techniques of projection operators. This approach can be used to substantiate the manipulation of the reducible and irreducible diagrams of the interaction representation, which is an alternative method of approach to the problem. The present arguments include the effects of an external force field.     

化合物SmCo5的电子结构、自旋和轨道磁矩及其交换作用分析

用自旋极化的MS-Xα方法研究了稀土-过渡族化合物SmCo5的电子态密度、自旋能级劈裂及原子磁矩.研究结果显示,由于化合物中Sm-Co间的轨道杂化效应,使Sm原子原来的5d0空轨道上占据了少量5d电子.由于Co(3d)-Sm(5d)电子间的直接交换作用,导致了Sm-Co间的磁性交换耦合,这是化合物中形成Sm-Co铁磁性长程序的一个重要原因.在SmCo5化合物中存在6个能级呈现负交换耦合,导致了SmCo5化合物的居里温度(与金属钴相比)明显下降.还研究了化合物中2c和3g晶位Co的自旋磁矩和轨道磁矩,发现Co(2c)晶位的自旋磁矩、轨道磁矩及冻结部分均略大于Co(3g)晶位,所以2c晶位的L-S耦合强度应略大于3g晶位.因此,2c晶位对SmCo5磁晶各向异性的影响也应大于3g晶位.但Co(2c)未被冻结的轨道磁矩却略小于Co(3g),所以两种晶位对磁晶各向异性的贡献大小之差别不应太大.考虑到4f电子的局域性和化合物中轨道杂化效应所导致的Sm(5d0)空轨道上占据了少量5d电子,可以得到Sm原子磁矩为1.29μB,与顺磁盐中Sm3+磁矩实验值(1.32-1.63uB)及金属中Sm原子磁矩实验值(1.74μB)基本符合.     

Spin and orbital magnetic moments of free nanoparticles

The determination of spin and orbital magnetic moments from the free atom to the bulk phase is an intriguing challenge for nanoscience, in particular, since most magnetic recording materials are based on nanostructures. We present temperature-dependent x-ray magnetic circular dichroism measurements of free Co clusters (N=8-22) from which the intrinsic spin and orbital magnetic moments of noninteracting magnetic nanoparticles have been deduced. An exceptionally strong enhancement of the orbital moment is verified for free magnetic clusters which is 4-6 times larger than the bulk value. Our temperature-dependent measurements reveal that the spin orientation along the external magnetic field is nearly saturated at ~20 K and 7 T, while the orbital orientation is clearly not.     

Determining electronic-band moments for complex molecular spectra

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 50, No. 4, pp. 631–639, April, 1989.     

A matrix method for the calculation of nonlocal exchange potential and spin-orbit coupling in the first-principles calculation method

Adequate treatment of exchange interaction and inclusion of spin-orbit coupling are important in obtaining reliable electronic structures of semiconductors and magnetic materials. Here, a matrix method has been derived to calculate nonlocal exchange potential and spin-orbit coupling within a muffin-tin-orbital based first-principles calculation method. Good agreement between calculated and experimental band gaps for various zinc-blende and wurtzite semiconductors demonstrated that direct calculation of nonlocal exchange potential is feasible. This method may be an improvement over the currently used LSDA  +  U or LSDA  +  GGA  +  U method, which uses an adjustable U parameter, for wide-band-gap semiconductors and magnetic materials. In this method, the use of an extended Hamiltonian matrix, which contains off-diagonal matrix elements between up- and down-spin states due to spin-orbit coupling, enables mutual adjustment of the occupation of up- and down-spin states through atomic orbitals. In contrast, in the conventional spin-polarized calculation method up- and down-spin sub-matrices are diagonalized separately. The calculated spin-orbit splitting for the triplet states at the Γ point for GaAs and ZnS are in good agreement with experimental data.